After recent developments there are miniature spectrometer modules available, which are based on a fibre optic input connection supplying light to a spectrograph, with optics based on a diffraction grating; and delivering optical signals to a linear detector array for detection. One example of these spectrometer modules is presented in U.S. Pat. No. 5,159,404. Thanks to parallel detection of wavelengths these spectrometers have become popular tools for real time analytical measurements. However, this technology has been limited to shorter wavelengths where suitable, high performance linear detector arrays are available (Si-detectors λ<1100 nm, InGaAs-detectors λ<1600 nm) and where high quality fibre optics is available (quartz λ<2300 nm). On the other hand there is large amount of analytical information available at the longer near infrared (NIR, up to 2500 nm) and IR wavelengths (>2500 nm), which could be exploited for real-time process control and quality monitoring, when high performance and cost effective instrumentation becomes available. Fibre based spectrometer solutions have been proposed for the longer infrared wavelengths, too, but available infrared fibres are expensive and mechanically fragile.
Linear variable filters (LVF) are another method for implementing spectrometers optics, which can provide wavelength separation for linear detector array based spectrometers. U.S. Pat. No. 5,166,755 discloses a spectrometer apparatus comprising image transfer optics, or a lens system, a shutter, an opto-electronic array of photosensitive elements, and a continuous linear variable filter located in overlaying relationship with the array. Line and area arrays are presented as options for the photosensitive array. When exploiting the latter option the two dimensions of the array are called the wavelength axis and the spatial axis, in which case the invention may be used as an imaging radiometer for space applications. Another application of the invention is illuminated by a separate light source aiming for use as a spectroreflectometer. However, this system is not suitable for on-line industrial measurements, which often require monitoring of fast moving non-homogeneous material. In such application various pixels along the wavelength axis would be imaged on different points of the moving sample. This situation would be seen by the user as significantly increased noise in the measured spectra. Furthermore, this invention does not present a solution, where cooled lower noise photosensitive arrays may be used, again very important for high speed on-line applications especially at IR wavelengths.
WO 01/31304 discloses an integrated optics probe for spectral analysis, aiming for applications that require spectral measurements of larger sample areas e.g percentage concentrations of agricultural products as well as colorimeter analysis of samples such as wallpaper. The presented apparatus comprises a polychromatic light source disposed in a first chamber arranged to irradiate a sample with a large illumination spot size, a wavelength separator disposed in a second chamber separating received light of different wavelengths and a detector with a wide viewing aperture, also disposed in the second chamber and positioned to receive light from the wavelength separator for detecting intensities at multiple selected wavelengths. A linear variable filter is proposed for the wavelength separator and a linear detector array for the detecting device. In a preferred embodiment there are separate windows for both the illuminating and the detecting chambers in order to implement a construction, where stray light from the lamp is not received by the detector from within the detection apparatus itself during a sample measurement. Furthermore, a diffuser is proposed in the detecting chamber in the path of the light received from the irradiated sample to ensure that only spectral information is measured without imaging of the sample. This invention is not optimal for on-line measurements requiring high spatial resolution, high measurement speed and simultaneously high signal-to-noise ratio. Illumination of a large area on the sample leads to reduced radiance or brightness on the sample and eventually lower signal-to-noise ratio. There are also high optical losses in the detecting chamber, because only a small portion of the light reflected from the sample enters to the wavelength separation and detection devices. Use of a diffuser would produce further losses in the detected signals, all these features leading to compromised signal-to-noise performance especially when high speed i.e. short integration time is required, which is often the case for on-line measurements serving process automation purposes.
U.S. Pat. No. 6,505,775 discloses a produce data collector with enhanced LVF spectrometer aiming for identifying produce items not labelled with bar codes in connection with a product checkout device. A typical spectrometer according to this invention comprises a linear variable filter splitting incoming light into a number of portions having different wavelengths; a photodetector adjacent the linear variable filter sampling the light portions and producing electrical signals containing information; an optical slit member with a width sufficient to minimize scattering by the interior surfaces of the linear variable filter; and a filter above the optical slit member filtering out light which is outside a wavelength range of operation. This invention has several drawbacks, too, if considered for on-line spectroscopy on a sample web or strip moving at a distance: There are no means for effective homogenisation of the measured radiation, in which case sample movement and distance variations may cause artefacts in the measured spectra. Furthermore there are no means to stabilize the wavelength axis and the measured signal levels against thermally induced variations in the linear variable filter and the linear detector array. Also signal level and signal-to-noise ratio is reduced, because of optical collection losses i.e. illumination radiance present on the sample is not imaged directly on the linear variable filter and the linear detector array.
EP 1 498 708 A1 discloses a small packaged sensor unit for non-destructive inspection of an object for an interior quality (including the ingredients) aiming inspecting fruits, vegetables, plant leaves, fish, meat etc. The sensor unit receives, through an optical fibre bundle, light emitted from an inspection light source and diffuse-transmitted through an inspection object, separates the received light spectroscopically into spectra, and inspects the spectra by an array type sensor for interior quality of the inspection object, wherein a light diffuser, continuous variable interference filter and a photoelectric conversion element are provided after the fibre bundle. Furthermore, fibres in the optical fibre bundle are twisted together to uniformize irregularity in the received light. In the preferred embodiment there is also a light diffuser constituted of optical glass which diffuse-reflects therein the light introduced from the light emitting end of the fibre bundle and emits it on the side opposite towards the continuous variable interference filter. This invention is however not optimal for measurements at a distance i.e. on-line applications, because the light received in the fibre bundle rapidly reduces with increasing measurement distance. On-line applications require that analytical results are not affected by distance variations to the passing sample flow. Furthermore, due to the fibre based design, this sensor is best suitable for shorter wavelengths transmitted by quartz fibres, but less applicable to longer (>2300 nm) wavelengths requiring infrared fibres.
U.S. Pat. No. 6,420,708 B2 discloses a spectroscopy analyzer using a detector array for use in measurements exploiting attenuated total reflection (ATR) technique. A typical implementation of this invention comprises an elongated light source, a sample stage, a device for producing a spectrum and an array of photosensitive elements. In the preferred embodiment an ATR crystal serves as the sample stage and a linear variable filter as the device for producing the spectrum. ATR technique allows measurements at infrared wavelengths too, but it is limited to samples brought to contact with the ATR crystal, e.g. liquid measurements. Therefore this invention is not suitable for on-line measurements at a distance from the passing sample web or strip.
WO 2004/013621 discloses a device for IR-spectrometric analysis of a solid, liquid or gaseus medium. A process probe according to this invention comprises at least one light source, at least one light wave guide connected to a sample, a linearly-variable filter, at least one detector and a regulator/analytical unit. During measurement operation the light is introduced into a defined region of the linearly-variable filter, subsequently the detector is moved into different locations relative to the linearly-variable filter and the regulator/analytical unit determines the spectrum of the medium from the measured values provided by the detector element. According to the principle of operation, this spectrometer records a spectrum in a scanning operation over a period of time. Therefore the spectrometer is not optimal for measurements on fast moving non-homogeneous materials, because different wavelengths are recorded on different points of the sample, which creates “moving-sample noise” in the measured spectra.
The background art summarized above does not meet requirements for typical on-line measurements for process automation applications. It is the purpose of this invention to present a spectrometer solution which                may be used at a distance from moving sample web or sample transport (on-line), while avoiding artefacts in the spectra due to sample movement and distance variation        allows good spatial resolution i.e. small measurement area        allows high measurement speed while maintaining high signal-to-noise ratio        provides possibility to use thermoelectrically cooled detector arrays for low noise detection        avoids wavelength limitations of fibre coupled spectrometers        avoids both signal and wavelength drifts due to variable operating temperature.        